Ok, Peter cleared up the more general doubt about exchangeability of the lek anywhere outside Albania. As to my original / more specific question:

Is there any place in Helsinki where I can change those back into
euros?

Seems like no, not in Helsinki.

A "Money Change" booth I stumbled upon in Kamppi Centre: no (raising a suspicious eyebrow at the bills I procuced).

At Forex, they also said no. "Not yet, but maybe some day, if and when the currency becomes more accepted internationally." The clerk also said she doubts any banks would change leks either. (According to her, Forex has the widest selection of currencies available across any establishments that do currency exchange here.)

Just to make sure, I asked a friend who works in a large bank. She checked with their travel currency department, and confirmed that the Albanian lek is not accepted.

So if I really needed to get rid of the leks in Finland, I guess I should post to Pallontallaajat travellers' forum in hope of finding someone going to Albania. (Dunno about that; the amount I have is so small it doesn't warrant going to much trouble.)

Maybe we can deduce that accepting Albanian money is currently limited to countries near Albania...